This invention relates to a steering axle for vehicles, being of a type which comprises:
a beam arranged for conventional mounting to the load-bearing structure of a vehicle; PA1 a road wheel hub supported rotatably on each of the two opposite longitudinal ends of the beam; PA1 a steering actuator arranged to act on at least one of said road wheel hubs; and PA1 a steering drive interconnecting the two road wheel hubs to synchronize their relative angular displacements in accordance with pre-determined drive parameters, said steering drive comprising two shafts, synchronously counter-rotating relatively to each other and being rotatively connected to the corresponding one of the hubs through respective angle transfer gear mechanisms, said transfer mechanisms including first and second gears keyed respectively to the end of said shaft next to the corresponding hub and being made rotatively rigid with said hub.
An axle with the above features is known from JP-A-60-1075.
The axle shown in that document is provided with a gear-type steering system operated through a steering actuator for pivoting the road wheel hubs fitted to the opposite ends of the beam.
That actuator is attached to the load-bearing structure of the vehicle and has an output shaft which carries, keyed to its free end, one gear of the steering system. The shaft extends through a bore in the load-bearing structure of the vehicle and a corresponding tubular passageway formed in the respective journal formation on the beam.
A steering axle to that design has a number of drawbacks.
First, the actuator is mounted at a location where it is subject to shocks and may come to harm. This is more likely to occur when the axle is used on such all-terrain vehicles as agricultural tractors, where the probability of shocks is relatively high.
Secondly, with the above construction, relative rotation is allowed to take place both between the actuator shaft and the corresponding journal formation, as well as between the latter and the load-bearing structure of the tractor whereto said actuator is, in turn, attached. The corresponding journal formation of the beam intervenes, therefore, between the actuator shaft and the load-bearing structure of the vehicle, while being mounted to swing with respect to either. Accordingly, the machining tolerances for the different component parts add algebraically to one another, to establish operating conditions that make the required oil-tight fits between the axle beam and the actuator shaft difficult to achieve. This problem is made the more serious by that the standard admissible tolerances for journal formations in the agricultural tractor industry are comparatively large. Further, with that design, the axle tends to steer, even with the steering wheel held still, every time that it is caused to perform swinging movements relatively to the vehicle structure, and this on account of the steering system gears being forced to roll about the actuator gear whenever such a situation of relative oscillation is encountered. Thus, the structure proposed in said document is unsuited to such applications.
It should be also noted that in the axle according to the Japanese Patent the drive which connects the axle shaft to the road wheel hub to drive the latter is of a type comprising a bevel gear pair. This drive arrangement increases the vertical dimension of the axle in the area of swivel connection to the road wheel hubs.
A further consequence of this prior structure is that, as a torque is transferred between the axle shaft and the wheel hub, radial thrust forces appear between the bevel gear pair. The radial thrust is discharged, on the axle shaft side, to a specially provided support which carries, at a location proximate to the corresponding gear, the axle shaft and the corresponding coaxial tubular shaft from the axle steering drive. The structure shown has a first bearing between the axle shaft and the tubular shaft of the steering drive, and a second bearing between the tubular shaft and a supporting flange rigid with the axle beam. It follows that the aforesaid radial thrust will be discharged to the support through the two bearings and the tubular shaft. Due to the frictional resistances that arise unavoidably from the above situation of radial loading, the torque being transferred between the axle shaft and the wheel hub makes an undesirable effect felt on the steering drive. This effect is the more apparent the higher is the torque being transferred through the axle shaft.
Other known axles to more traditional designs are equipped with a steering drive which includes a quadrilateral linkage effective under steer to cause the centers of rotation of the steering axle wheels, respectively on the turn inside and outside, to be coincident. This arrangement has substantially the shape of an isosceles trapezoid whose major base is formed by the axle section included between the two stub axles for the wheel hubs, oblique sides are the steering arms rigid with the respective hubs, and minor base is a rod tying the steering arms together.
With that quadrilateral linkage configuration, the centers of rotation are brought to coincide when the extension lines from the steering arms intersect approximately on the centerline of the rear axle. It follows that this alignment will be dependent on several factors, such as the vehicle wheelbase, tread (as a function of the stub axle spacing), etc. In many cases, however, an axle is manufactured without all the parameters of its ultimate utilization being known, or is intended for installation on vehicles having different parameters, or is of an adjustable type whose adjustments affect the steering geometry. Such axles cannot ensure, therefore, an appropriate steering geometry for every installation and/or adjustment, e.g. every tread adjustment.
In addition, axles of that kind are comparatively bulky constructions due both to the steering arms and the tie rod provided. The intrusive steering arms, by interfering with structural members of the vehicle, may become a limiting factor of the maximum steering angle afforded. Further, the steering arms and tie rod are both subject to shocks and the actions of such environmental agents as dirt, mud, and dust, all apt to deteriorate them over time.
To obviate in part the problems posed by the space requirements, axles are employed which have steering actuators of the double-acting, two-rod hydraulic jack type mounted close against the box-type structure of the axle. The jack rods are swivel-associated with their respective steering arms through struts. This construction, however, makes for inaccurate steering geometry, which frequently results in a small, but perceivable, amount of tyre slip in the radial direction on a turn.